The invention relates an alkali-free aluminoboro-silicate glass, for example for use in display technology and other applications, and to a process for its preparation.
The basic requirements made on a glass which is to be used in display technology, for example as the front screen for a flat-panel display, have been described in detail by W. H. Dumbaugh, P. L. Bocko and F. P. Fehlner (xe2x80x9cGlasses for flat-panel displaysxe2x80x9d in xe2x80x9cHigh-Performance Glassesxe2x80x9d, ed. M. Cable and J. M. Parker, Blackie and Son Limited, Glasgow and London, 1992). The glasses currently available for TFT (thin film transistor) applications have also been reviewed in the article xe2x80x9cAdvanced glass substrates for flat panel displaysxe2x80x9d by J. C. Lapp, P. L. Bocko and J. W. Nelson, Corning Research 1994. The quality of the properties which are needed for flat glass substrates and have a decisive effect on the precision of the imaging properties of a system is dictated, on the one hand, directly by the composition of the glass and, on the other hand, by the production, processing and shaping methods and their ability to set particular properties for the glasses, for example thickness profile parameters and planarity parameters, the applicability of the methods being frequently limited in turn by the composition of the glass or by properties of the glass.
Borosilicate glasses play a dominant role in a large number of technically oriented glass applications. In particular, they are distinguished by high stability when subjected to cyclic and differential thermal loads, by low thermal expansion and by good resistance to corrosive reagents and media.
Borosilicate glasses are therefore in principle also of interest for use as substrate glass in display technology, but the display production process, for example for thin-film active matrix liquid crystal displays (TFT-AMLCDs), and the desired application require a very specific property profile of the glasses:
a coefficient of thermal expansion xcex120/300 matched to polycrystalline silicon of from 3.0 to 3.8xc3x9710xe2x88x926/K; in particular an xcex120/300 of between 3.0 and 3.3xc3x9710xe2x88x926/K means a good expansion match even up to temperatures of up to 700xc2x0 C.
a temperature at a viscosity of 1014.5 dPas of at least 680xc2x0 C. in order to ensure high thermal processing and dimensional stability of the glass during production, in particular low compaction of the glass in the cooling phase,
an alkali-free glass composition, a maximum alkali metal oxide content of xcexa3 R2O=2000 ppm being tolerable, in order to avoid poisoning the microstructured thin-film transistors by alkali metal ions diffusing into the semiconductor layer,
a high high-temperature process stability documented by a transformation temperature Tg of between 710xc2x0 C. and 780xc2x0 C.,
sufficient chemical, i.e. hydrolytic, acid and alkali resistance, with respect to the reagents and media used in the microstructuring process,
a very low density, i.e. xcfx81xe2x89xa62.50 g/cm3, in order to keep the overall weight of the display low in view of the trend towards large screen formats.
Furthermore, the visual quality of the glass screens, that is to say the quality in terms of the absence of crystalline inclusions, knots and bubbles, must be very good.
This complex and extensive requirement profile is fulfilled best by borosilicate glasses from the alkaline earth metal aluminoborosilicate glass subfamily. The known commercially available glasses for TFT-AMLCD applications belong to this type of glass; the glasses in the patents or patent applications discussed below are also representatives of this group. However, all currently known glasses for display applications still have disadvantages and do not meet the full list of requirements.
U.S. Pat. No. 5,506,180 describes thermally stable and chemically resistant glasses, amongst other things for use as TFT display glass. On account of the high B2O3 contents of 10% or more and the relatively low SiO2 levels (46-56%), glasses of this type are not sufficiently resistant to hydrochloric acid. Furthermore, their resistance to solutions containing hydrofluoric acid is only moderate. The matching of the thermal expansion to polycrystalline silicon is insufficient. The processing temperatures VA of less than 1150xc2x0 C. are too low to make it possible to use drawing methods such as the microsheet down-draw method and the overflow fusion method as alternatives to the float process. Furthermore, the specified strain points of 642xc2x0 C. or less are too low to ensure that there is little compaction.
European Patent EP 510 544 B1 describes alkali-free glasses which can be made by the float process and, amongst other things, are used as a substrate for a variety of displays and photographic masks. The glasses are free of BaO and MgO and have very low levels of B2O3. However, a disadvantage of these glasses is that their alkaline earth metal levels are high, with CaO levels being at least 10% by weight and SrO levels being at least 11% by weight. Their thermal expansion of 4.5-6.0xc3x9710xe2x88x926/K is no longer sufficient to meet the requirements of high-quality TFT display glasses which are matched to polycrystalline silicon.
EP 527 320 B1 describes flat panel display devices having a strontium aluminosilicate glass substrate comprising at least 21 mol % of SrO. The glass compositions for glasses with high devitrification stability appear to be specifically designed to be suitable for being made in the overflow fusion draw method. The density of the glasses and their coefficient of thermal expansion are too high.
Japanese JP 8-295530 A describes alkali-free glass substrates whose stability with respect to hydrochloric acid will be low owing to the high B2O3 content (up to 15% by weight).
PCT application WO 97/11919 also describes alkali-free glass substrates. The glasses comprise relatively little SiO2 and little or no MgO. They may contain up to 5% by weight each of ZnO and TiO2. ZnO can cause glass defects owing to its tendency to evaporate from the glass surface in the float bath and subsequently condense. The possible high TiO2 content will cause a brown tint in the glasses when conventional raw materials are used, since the Fe3+ always present in the usual raw materials forms a brown colour complex with Ti4+. The same applies to the glass substrates described in WO 97/11920.
European Patent Application EP 714 862 A1 describes alkali-free ZrO2- and TiO2-free glasses for use in TFT flat panel displays. Aluminosilicate glasses of this type having relatively high SiO2 contents are highly viscous, making efficient refining extremely problematic. These glasses do not therefore meet the requisite high demands in terms of visual quality. EP 672 629 A2 or U.S. Pat. No. 5,508,237 describe aluminosilicate glasses for flat panel displays. They present a variety of composition ranges with different coefficients of thermal expansion. These glasses are allegedly processable not only by the overflow fusion draw method but also with other flat glass production methods. However, in particular the glasses which have a coefficient of thermal expansion matched to polycrystalline Si have very high processing temperatures VA, which make them unsuitable for the float process. As in the case of the glasses described above, the visual quality here is not high, since no way of effective refining, in particular one compatible with the float process, is presented. The refining agents Sb2O, and As2O3 mentioned by way of example are unsuitable for the float process because they can be reduced readily. The same is true for the optional glass components Ta2O5 and Nb2O5.
In the alkali-free glass substrates for TFT-AMLCD from JP 9-48632 A, the visual quality will likewise not be high, since merely SiO2, B2O3, MgO and BaO have to be present in the glass.
German Patent DE 38 08 573 C2 describes alkali-free SrO- and B2O3-free aluminosilicate glasses which contain SnO2, are easy to melt and can be refined at low cost. The glasses exhibit high chemical stability. They are used as photographic mask glass. At 4.0xc3x9710xe2x88x926/K, their thermal expansion behaviour is not optimally matched to that of polycrystalline Si. On account of the fact that they are free of B2O3, the glasses have a temperature/viscosity profile which is unfavourable for flat glass production processes.
German Patent DE 196 17 344 C1 in the name of the applicant company also describes alkali-free glasses which contain SnO2. The glasses have a relatively low SiO2 level and are free of TiO2. With a coefficient of thermal expansion of about 3.7xc3x9710xe2x88x926/K and very good chemical stability, these glasses are suitable for use in display technology. There is, however, still a need for improvement in terms of making them economically manufacturable with the float method as well as the draw method, that is to say in terms of being xe2x80x9cuniversallyxe2x80x9d manufacturable and in terms of lowering the thermal expansion and the density as is desired. This is also true as regards the TiO2- and SrO-free glasses described in German Patent DE 196 036 98 C1 in the name of the applicant company.
Alkali-free aluminoborosilicate glasses which have relatively low SiO2 contents and are very suitable as substrate glasses for current TFT displays have already been described in another German patent in the name of the applicant company, DE 197 39 912 C1. A need for improvement only exists in terms of the low density requirements which will increase in future owing to the increasing screen format sizes, and in terms of a very exact expansion match to polycrystalline silicon.
WO 97/30001 describes substrates for solar cells and TFTs. The substrates consist of glasses or glass ceramics which are based on an aluminosilicate glass and have a thermal stability of at least 700xc2x0 C. Their constituents and composition ranges may vary within wide limits and correspondingly vary in terms of their properties. The glasses may contain Cs2O. On account of the fact that they are free of B2O3, the glasses will have relatively high densities and an inadequate crystallization stability.
WO 98/27019 describes substrates for flat panel displays and photovoltaic devices. The substrates are comprised of glasses exhibiting a coefficient of thermal expansion xcex120/300 of between 3.0 and 4.0xc3x9710xe2x88x926/K and a temperature at 1014.5 dPas of more than 600xc2x0 C. and which consist of SiO2, Al2O3, B2O3 and alkaline earth metal oxid(es) (10-25% by weight of RO), where of these ROs only CaO has to be present and SrO+BaO is between 0 und 3% by weight. However, such an unbalanced ratio of small cations to large cations of the alkaline earth metals results in poor devitrification stability.
German Offenlegungsschrift DE 196 01 922 A1 describes alkali-free glasses which contain SnO and have a rather high B2O3 content and alkaline earth metal levels which may vary within wide limits.
JP 9-263 421 A and JP 10-454 22 A describe alkali-free glasses which can be processed by the float method for use as substrates in flat-panel display systems. The glasses listed have rather high temperatures at a viscosity of 102 dPas, which signifies poor meltability and makes low-cost production impossible, since the requisite temperature range also means that very high requirements are made of the tank and distributor material with respect to corrosion resistance. The glasses of JP 10-454 22 A are free of TiO2, ZrO2 and CeO2. The density of the BaO-containing glasses is relatively high at xcfx81 greater than 2.6 g/cm3. The glasses of JP 9-263 421 A preferably contain no BaO and are free of TiO2, ZrO2, CeO2 and SnO2.
Alkali-free glasses for use as substrate glass in displays are also described in JP 10-130034 A, JP 10-114538 and JP 10-59741 A. However, their constituents and composition ranges may vary within wide limits and correspondingly vary in terms of their properties. All the glasses may contain up to 10% by weight of ZnO which is unfavourable for production by the float process.
The glasses described in JP 10-130034 A contain at least 0.05% by weight of As2O3 and 0.05% by weight of SnO2. Owing to their As2O3 content, they cannot be made by the float process. Owing to the very low SiO2 contents (only about 55% by weight) of some of the exemplary glasses, the HCl resistance does not meet the requirements of flat glass and photovoltaics substrates.
JP 10-114538 A replaces the As2O3 refinement of JP 10-130034 A with an Sb2O2 refinement, and as a result the Sb2O3 content is between 0.05 and 3% by weight which likewise rules out production by the float process. JP 10-59741 A uses 0.05-2.0% by weight of SnO2 as the sole refining agent. This document does not mention any additions and/or components which stabilize the tetravalent tin dioxide until the refining. temperature is reached and prevent premature generation of oxygen. As a result, the glasses will not exhibit the high visual quality (freedom from bubbles) required for display applications.
It is an object of the present invention to provide glasses which meet the said physical and chemical requirements of glasses for substrates for TFT displays, for MEMS (microengineering and -mechanical systems), for wafer-bondable insulators (SOT, silicon on insulator) and for thin film solar cells, glasses which have a favourable processing temperature range and high devitrification. stability, so that a variety of flat glass manufacturing methods, such as the float method or draw method can be employed for producing them, depending on the specific requirement profile for the substrat types mentioned. The thicknesses which can be produced therefore also vary in the range between 30 xcexcm and a few mm. Glasses of this type need to be readily meltable and refinable.
The formation of highly volatile borate compounds such as zinc borate, lead borate and barium aluminoborate, which can impair the internal glass quality, should be avoided or at least minimized.
In order to produce microsheets in the thickness range between 30 and 50 xcexcm using the microsheet down-draw method (MDD method), the glasses should at the same time have very high devitrification stability and specific processing temperatures VA:
Suitable processing temperatures are temperatures at a viscosity of 104 dPas of preferably 1260 to 1320xc2x0 C. One characteristic for the devitrification stability or crystallization stability is the maximum crystal growth rate vMAX [xcexcm/h]. It indicates the largest observed growth length of the crystals which are formed: on a plot of the growth rate v of the crystals against temperature T, vMAX corresponds to the growth rate at the temperature of maximum crystal growth, at KGMAX. The smaller vMAX, the less crystalline volume is formed. This vMAX should in this case be no more than 10 xcexcm/h.
In order to produce glass panels for display applications using the microfloat method, in particular in large formats, the temperature at 104 dPas viscosity should preferably be between 1250 and 1350xc2x0 C. Readily reducible glass components such as As2O3, Sb2O3, P2O5, Bi2O3, Nb2O5, Ta2O5, PbO, CdO and ZnO should not be contained in the glass composition, because they can be reduced to the elementary state under the reducing conditions in the float bath, and can produce a grey metallic surface reflection or other microsurface defects. The requirements of crystallization stability are not as high here as in the aforementioned MDD method. A via of less than or equal to 30 xcexcm/h is therefore still sufficient.
The aforementioned object is achieved by a glass according to Patent claim 1 and by a process according to Patent claim 7.
According to the invention, the three glass-forming components SiO2, B2O3 and Al2O3 are present in narrowly defined contents, and thus within a narrow ratio relative to one another as well: the B2O3 content is rather low and is at least 6.5% by weight and at most 9.5% by weight. Limitation to less than 9% by weight is preferred. A B2O3 content of between 7 and 8.5% by weight is particularly preferred. The Al2O3 content can vary between 14 and 21% by weight. A content of at least 15% by weight and at most 20% by weight is preferred. A content of at least 15.5% by weight and at most 19.5% by weight is particularly preferred. Surprisingly, the SiO2 content can be relatively high, i.e.  greater than 60-65% by weight, without any disadvantages in terms of viscosity in the melting range. As a result, the chemical resistance (for example to 5% strength HCl) is significantly improved and a low density can be ensured at the same time. Even higher levels would lead to an excessive increase in the viscosity in the melting range. A level of at least 60.5% by weight is preferred, a content of at most 62.5% by weight is particularly preferred. In this way it is possible to achieve the desired low coefficients of thermal expansion xcex120/300 in the range of from 3.0 to 3.8xc3x9710xe2x88x926/K with at the same time a low density p of at most 2.50 g/cm3. The particularly preferred range of xcex120/300=3.0-3.3xc3x9710xe2x88x926/K is achieved by a high Al2O3 content of at least 18.0% by weight at an SiO. content of at least 60.5% by weight. On account of the mutual influence of B2O3 and Al2O3 in the glass structure, the desired good chemical and crystallization stability can only be achieved in the aforementioned narrow range of B2O3 content. Lower B2O3 contents make the glass more susceptible to devitrification and increase the thermal expansion by directly affecting the Al coordination, with the number of aluminium atoms with coordination numbers 5 and 6 increasing, If the B2O3 and Al2O3 content is too high, the resistance to hydrochloric acid is reduced.
The glass of the invention contains a relatively low amount of alkaline-earth metal oxides. This results in a low density, a high strain point and a low thermal expansion. They are present in the glass at a total of at least 8% by weight. At an even lower level, the temperatures at the viscosities required for melting and shaping would be too high. A balanced ratio between small cations and large cations of the alkaline-earth metal oxides has a positive effect on the processing temperature and on the devitrification stability. The glass therefore contains 1-8% by weight of MgO and 1-6% by weight of CaO and 0.1-3.5% by weight of BaO and 1-9% by weight of SrO. The total level of MgO, CaO, SrO and BaO should in this case remain restricted to a maximum of 16% by weight, since otherwise the chemical resistance again decreases.
The use of SrO is preferred over BaO in order to keep the density of the glasses low. If the heavy oxides BaO and SrO were fully eliminated, or if their level were too low, then the glass would become more susceptible to devitrification and the tranformation temperature as well as the temperature at a viscosity of 1014.5 dPas would decrease. At excessively high levels of BaO and SrO, the processing temperature would become unacceptably high. Glasses which are low in BaO are preferably processed by the float method, while glasses containing a higher level of BaO are preferably processed by the draw method because of their better crystallization stability. Preference is given to an alkaline earth metal content (xcexa3RO=MgO+CaO+SrO+BaO) of between 9 and  less than 15% by weight, where the individual oxides should be present at the following levels: MgO 1-7% by weight; CaO 1-5% by weight; SrO 2-8% by weight; BaO 0.5-3% by weight. Particular preference is given to 3-5% by weight of MgO, 2-5% by weight of CaO, 3-7% by weight of SrO and 0.6xe2x88x92 less than 3, very particularly preferred up to 1.5% by weight of BaO, at 10-14% by weight of RO. Apart from unavoidable impurities, the glass is free of ZnO and alkali metal oxides. On account of the special content of large cations (Ba2+, Sr2+) in the borosilicate base glass at relatively high Al2O3 contents, the galsses of the invention exhibit a low thermal expansion and very favourable viscosity/temperature profiles, i.e. a steep viscosity profile in the transformation range and a flatter viscosity increase in the processing range.
The glass furthermore contains 0.1-1.5% by weight of ZrO2. ZrO2 improves chemical resistance. A minimum content of 0.2% by weight is preferred. The maximum ZrO2 content is limited by its low solubility and is preferably 1% by weight, particularly preferably 0.6% by weight. The glass also contains 0.1-1% by weight of TiO2. This minimizes the otherwise often observed susceptibility of aluminoborosilicate glasses to solarization, that is to say reduction in transmission in visible wavelengths because of UV-VIS radiation. The TiO2 content is preferably at most 0.5% by weight, particularly preferably at least 0.2% by weight and at most 0.4% by weight.
Furthermore, the glass contains tin oxide which is present in the glass in the redox equilibrium SnO2/SnO and acts as a refining agent, in an amount of 0.1-1.0% by weight, calculated and used as SnO2. Preference is given to a content of at least 0.2% by weight and at most 0.8% by weight, particularly preferably at most 0.6% by weight.
Besides the aforementioned SnO2 content, the presence of 0.01-1.0% by weight of CeO2 is essential to the invention: By the combination of SnO2 with CeO2, the SnO2/SnO redox equilibrium is stabilized and a refining effect is achieved which is exceptionally good for aluminoborosilicate glasses, so that the glasses according to the invention exhibit the requisite high visual quality. Furthermore, SnO2 and CeO2 in combination with ZrO2 stabilize the chemical resistance of the glasses. However, at higher CeO2 contents, the UV-absorption increases and the absorption cut-off shifts into the VIS range, resulting in the occurrence of a distinct yellow tint in the glass. At the same time, the glasses exhibit a notable fluorescence. A content of at most 0.5% by weight is therefore preferred.
This relatively low CeO2 content is made possible by the addition of nitrate which sufficiently stabilizes the SnO2/SnO redox equilibrium.
According to the invention, NH4NO3 is added to the batch in amounts of between 0.2 and 3% by weight. The strong oxidizing effect of NH4NO3 stabilizes the refining agent combination SnO2/CeO2 in the melting region in its tetravalent form so that a premature oxygen release is avoided.
As a result, a significantly higher proportion of tetravalent SnO2 and CeO2 is availabe for decomposition into SnO and Ce2O3 with release of oxygen (=refining) on reaching the refining temperature, so that an exceptionally good visual quality of the glasses was observed.
Preference is given to adding at least 0.5% by weight of NH4NO3, particularly preferably at least at least 1.0% by weight.
By the addition of NH4NO3, other polyvalent ions in the glass melt can be oxidized at the same time. For instance, the very good transmission of the glasses is, amongst other things, a result of the oxidation of Fe2+ to Fe3+ which causes much less colouration in the glass than its divalent form. NH4NO3 completely decomposes at these high temperatures so that the glass properties are not impaired by any residues.
Amongst other things, because it is thus possible to eliminate the use of the refining agents arsenic oxide and antimony oxide, and the glasses are free both of these components and of the other readily reducible constituents lead oxide, cadmium oxide, zinc oxide, bismuth oxide, niobium oxide, tantalum oxide and phosphorus oxide, apart from unavoidable impurities, these glasses can be processed not only using a variety of draw methods, but also by the float method. If the latter method is not to be employed, the glass may contain up to 1.5% by weight of As2O3 and/or Sb2O3 as additional refining agent(s) under nonreducing conditions, for example in the down-draw process. It is also possible to add 1.5% by weight each of Clxe2x88x92 (for example in the form of BaCl2 or NH4Cl), Fxe2x88x92 (for example in the form of CaF2) or SO42xe2x88x92 (for example in the form of BaSO4). The sum of As2O3, Sb2O3, Cxe2x88x92, Fxe2x88x92 and SO42xe2x88x92 should, however, not exceed 1.5% by weight. By NH4Cl addition, it is possible to reduce the water content of the melt even when relatively water-rich raw materials (such as Al(OH)3 or Mg(OH)2, which in turn improve the melting behaviour) are used, avoiding reboil problems during settling and conditioning of the refined glass and making it easier to achieve the desired high visual glass quality (bubbles, inclusions etc.).